A seismic cable will generally have sensor modules distributed along its length. Inside the sensor module is one or more sensors. Examples of sensors are geophones, accelerometers, hydrophones, tilt meters, magnetometers. The sensors can include electronics conditioning the signal and/or digitizing it. The sensors can be connected by leads, transmitting the sensor data through the seismic cable to electronics modules located along the cable or between cable sections, or can be connected by a data bus.
The seismic sensors are intended to be disposed, in use, on the earth's surface. The term “earth's surface” as used herein includes the sea-bed, land, and the transition zone. When the sensor is disposed on the earth's surface, the coupling of the seismic sensing element(s) to the earth is commonly provided in the prior art by a housing of the sensor; the housing also provides physical protection for the sensing element(s). In the case of sea-bed seismic data acquisition, the cable is then lowered onto the sea-bed to deploy the sensors at their desired locations on the sea-bed.
Seabed or ocean bottom cable systems generally are designed to meet two conflicting goals. First, the cable system must be robust and resistant to damage. For example, the cable system must survive and operate at great water depth. Also, the cable system may be roughly handled during deployment and retrieval. Second, the cable system should be sensitive to acoustic vibrations and not compromise the quality of data recorded by the sensor units. To design and construct a robust but sensitive cable requires balancing robustness and sensitivity through a large number of tradeoffs.
Sea-bed seismic sensors generally record the pressure and the elastic wavefield of the seismic data. The pressure is a scalar quantity, whereas the elastic wavefield is a vector quantity and it is therefore necessary to measure the components of the elastic wavefield in three non-coplanar directions. The three directions chosen are usually the x-direction (defined as being parallel to the cable, and also known as the “in-line” direction), the y-direction (defined as being perpendicular to the cable, and also known as the “cross-line” direction), and the z-direction (vertical). In total, therefore, four components of the seismic data are measured. Four-component seismic data recording at the sea-bed has proven to be a very successful method for imaging through gas saturated overburdens and for characterizing hydrocarbon reservoirs through lithology identification and fluid discrimination. The 3-component data for the elastic wavefield are especially useful, since they enable the separation of the P-waves from the shear S-waves.
Reliable interpretation of the elastic wavefield is possible only if the three components of the wavefield are recorded accurately. Seafloor multi-component recording systems available to the market today have problems meeting this objective. The principal problem that arises is that robust cables are stiff and acoustically couple too well to the sensor unit, thus limiting pick up from the seabed below the sensor unit. As a result it is commonly necessary to sacrifice either robust cable construction or high quality seismic recording.
It is thus a desire to provide a seismic cable systems that addresses drawbacks of the prior art systems. It is a further desire to provide a seismic cable system that provides a robust cable construction and high quality seismic recording. It is a still further desire to provide a sensor that is substantially acoustically decoupled from a mechanically coupled, robust, cable.